Collate rails_requests_complete_seconds over controller dimension as well

gitlab-cookbooks/gitlab-mtail!57 (closed) will add a controller dimension/label to the rails_requests_completed_seconds histogram but we want the various collations over that histogram to summarize all values in that dimension, not create a whole bunch of new metric instances. Related to alerts in rules/gitlab-com-latencys.yml.

NB: Adding the exclusion for controller="HealthController" in error-rates.yml will follow in a separate MR, once 57 above is merged.